a) Field of the Invention
The invention is directed to a method for obtaining image information, for example, about the surface configuration or surface structure of the palm or inside surface of a hand, the edge of a hand, the extended four fingers or an extended thumb, wherein the object to be recorded is placed on a scanning surface, an image of the surface structure is recorded with an illumination beam path directed onto the scanning surface and reflected by the scanning surface, and this image is subsequently projected onto the reception surface of an optoelectronic image converter. The invention is further directed to an arrangement for carrying out the method.
b) Description of the Related Art
Capture of impressions for the traditional formats consisting of rolled individual fingers, flat, plain (not rolled) four extended fingers of the hand which are to be captured simultaneously, and flat, plain (not rolled) extended thumb is known in the art. The size of the scanning surface is typically about 40 mmxc3x9740 mm for rolled individual fingers and about 75 mmxc3x9758 mm for flat simultaneous impressions of the four extended fingers. The optoelectronic scanning devices are adapted to these formats.
The capture of impressions of the kind mentioned above or of the pattern of papillary lines is based on the fundamental optical principle of frustrated total reflection. In known optoelectronic processes and arrangements, this basic principle is applied essentially by adapting to the specific conditions of the available image converters which usually take the form of CCD cameras. At the same time, this also results in the need for adapting to the limited quantity of light-sensitive sensors available in the image converter in the horizontal and vertical directions with respect to optical requirements.
DE 34 21 220 C2 describes a device for the capture, examination and identification of fingerprints in which a distortion-free optical imaging of the fingerprint is carried out on the light-receiving surface of a TV camera. The aspect ratio of the finger scanning surface is transmitted unchanged to the sensor of the TV camera by means of the special optical arrangement wherein deflecting prisms for anamorphotic magnification of the fingerprint and for compensation of astigmatism are provided between the reflecting prism and the image-side end part of the device.
It is disadvantageous in this arrangement that the need to provide a distortion-free optical arrangement can result in prisms with very small angles which are technologically difficult to produce and whose use in the device, moreover, leads to problems with respect to adjustment and image quality because of very strict permissible tolerances.
U.S. Pat. No. 5,650,842 describes an optical arrangement which is directed exclusively to the recording of four fingers simultaneously. The rectangular format of the finger scanning surface is imaged on a CCD camera with an extreme aspect ratio of about 1.6:1 by a special optical arrangement in such a way that the image matches the aspect ratio of the sensor surface by approximately 1.33:1. This strong optical distortion in the horizontal direction by a factor of 0.831 is compensated subsequently during readout of the analog image signals prepared by the CCD camera with oversampling by a factor of 1.203.
This procedure is disadvantageous in that, in order to obtain a clean signal shape, the analog signal must be sharply defined in its bandwidth so that no interference occurs in the oversampled digital image signal. This has a negative effect on optical resolution in the horizontal direction. Also, in the above-cited reference the imaging beam path is guided away from the finger scanning surface in the direction of the vertical axis of the image. However, the device housing must therefore be disadvantageously increased in size toward the front, i.e., toward the user, and a vertical closing surface must be provided, which greatly encumbers accessibility particularly when recording the flat extended thumb, or even renders capture impossible in the case of persons with limited articulation of the fingers.
An essential requirement in the capture of fingerprints, above all in law enforcement entities, is to generate images of the highest optical quality so as to provide the foundation for the input and use of the images in automatic fingerprint and hand-impression identification systems and to ensure a high rate of hits in the comparison with images which are already stored.
For this reason, spatial resolution may not fall below a minimum and geometric distortion may not exceed a maximum. A high spatial resolution is important for clear, sharp visibility of even fine structures in the papillary lines; reduced distortion offers the assurance of definite determination of the characteristic features, the minutiae, as they are called. Further, a good image contrast is a further prerequisite.
There is also an increasing desire on the part of users of identification systems of the type mentioned above to be able to capture images of the inside surface, the edge of the hand, the extended thumb, the extended four fingers, etc. with only one device and also with one and the same scanning surface present in this device, so that it is no longer necessary to have available a plurality of different application-specific devices in order to carry out all of the tasks performed by entities concerned with identification.
The solutions available in the prior art up to this point are not suitable for comprehensively meeting the above-mentioned requirements.
Proceeding from the prior art, it is the primary object of the invention to further develop a method of the kind described above in such a way that the flat or plane image of the inside surface of a hand and/or the edge of a hand and/or the extended four fingers and/or extended thumb is generated by one and the same scanning surface with high image quality and in a comfortable manner for the person in question.
According to the invention, this object is met in that the image of the surface structure is recorded with an aspect ratio BA:HA less than 1, wherein the fingers of the hand placed on the scanning surface are oriented parallel to height HA, the aspect ratio of the image is changed during the optical transmission from the scanning surface to the optoelectronic image converter, wherein the height is compressed in such a way and/or the width is expanded in such a way that an image which is optically distorted to the aspect ratio of BE:HE greater than 1 reaches the reception surface, the optically distorted image is then converted into electronic image information, wherein exactly one analog or digital value is obtained for every output signal of an individual sensor, and the electronic image information then undergoes a calculated or computational change in the aspect ratio, wherein the height is computationally expanded and/or the width is computationally compressed until the original aspect ratio BA:HA less than 1 has been restored.
In this way, it is possible to utilize a scanning prism whose scanning surface is sufficiently large to capture the entire inside surface of the hand. By means of preferably anamorphotic optics arranged after the scanning surface, the aspect ratio of the optical image recorded by this scanning surface is distorted in a defined manner by expanding and/or compressing and is directed via an imaging objective to the reception surface (for example, of a CCD camera), where the optical image information is converted to analog electronic image information, wherein, according to the invention, the aspect ratio is not changed.
Subsequently, the analog electronic image information is digitized. In contrast to the prior art, this is done in such a way that exactly one digital value is formed for every unit of analog image information read out of an individual light-sensitive CCD sensor. Accordingly, an image point-synchronized digitization is carried out while maintaining the distortion with which the image impinges on the reception surface.
The image which is now in digital form is compressed in the direction in which it was formerly optically expanded and/or is expanded in the direction in which it was formerly optically compressed, wherein the compression factor exactly compensates for the defined distortion of the image aspect ratio brought about by the anamorphotic optics. This results in a digital, distortion-free image of the papillary lines of the hand.
The advantage consists in that it is possible to record with high optical resolution the plane image of the inside surface of a hand and/or the edge of a hand and/or the extended four fingers and/or the extended thumb with only one camera and with optical components that are fixed with respect to the device. The resolution which can be achieved in this way not only satisfies, but exceeds the quality standard of 500 pixels/inch currently demanded for application in law enforcement identification entities, for example.
In an advantageous development of the invention, the image is captured by a scanning surface with an aspect ratio of BA:HA=0.92 and the aspect ratio of the image is changed in the course of optical transmission from the scanning surface to the sensor reception surface to an aspect ratio of BE:HE=1.04. The change in the aspect ratio of the image on the way from the scanning surface to the reception surface is preferably carried out by means of anamorphotic optics or an anamorphotic optical subassembly comprising three prisms and an objective.
In a preferred development of the invention, the compression of the height and/or the expansion of the width of the digitized image is carried out computationally by interpolation. The image can then be read out visually via a monitor or stored as a data record and reused at a later time.
In contrast to the prior art, apart from the differences in the manner of optical distortion of the image in the beam path between the scanning surface and the sensor reception surface, the optical distortion is not compensated by oversampling the image information present at the sensor output, but rather an image point-synchronized digitization is first carried out and then the quantity of image information is reduced computationally, preferably by interpolation, and, therefore, the resolution is adapted to the required target resolution.
Further, it is provided in a development of the invention that the plane image of the fingerprint or hand-impression is obtained by means of the principle of frustrated total reflection. In this connection, the plane image is obtained from the points on the scanning surface which are in contact with the raised structures of the hand or fingers.
Alternatively, the image can also be obtained from the diffuse reflection of the illumination light of the portions of the scanning surface which are in contact with the raised structures of the hand.
It is a further object of the invention to provide an arrangement which does not have the disadvantages of the arrangements known from the prior art and which achieves a high optical resolution in both coordinates of the plane image.
According to the invention, this object is met in that the scanning surface and the captured image of the surface structure have an aspect ratio of BA:HA less than 1, in that at least one optical subassembly influencing the aspect ratio of the image is provided in the illumination beam path reflected from the scanning surface, wherein an optical expansion of the width and/or an optical compression of the height and accordingly an optical distortion of the image to an aspect ratio of BE:HE greater than 1 is carried out by means of this optical subassembly, in that the individual sensors of the optoelectronic image converter are followed by analog-to-digital converters for converting the output signals into exactly one digital value in each instance and by a computing unit in which the width is computationally compressed and/or the height is computationally expanded and, therefore, a rectification of the image to the original aspect ratio BA:HA less than 1 is carried out.
In an advantageous manner, aspect ratio BA:HA=0.92 and aspect ratio BE:HE=1.04. Accordingly, it can be provided, for example, that the scanning surface is constructed with a width BA of 120 mm and a height HA of 130 mm and the optoelectronic image converter is a CCD camera which has a reception surface with an aspect ratio of BE:HE=1.04 or in which a reception surface with an aspect ratio BE:HE=1.04 can be utilized.
The optical subassembly influencing the aspect ratio of the image is preferably constructed as anamorphotic optics with two prisms and an objective. This enables a simple construction of the device arrangement with uncomplicated structural component units which are simple to manufacture in terms of technology.
In a particularly preferred development of the invention, the reception surface of the CCD camera is formed of 3072xc3x973072 individual sensors which are arranged in a two-dimensional raster, wherein the raster spacing between the individual sensors is identical in both dimensions. A partial surface of the sensors with the above-indicated aspect ratio BE:HE=1.04 can be used, for example.
The arrangement according to the invention is further developed in such a way that the outputs of the individual sensors are applied, via analog-to-digital converters, to the computing unit which has a digital computing circuit for compressing and/or expanding the quantity and values of the image signals in both coordinates of the plane image by interpolation.
In another advantageous development of the arrangement according to the invention, the radiating direction of the illumination beam within a scanning prism is inclined at an angle xcex1 toward the scanning surface, which makes it possible to obtain the image according to the principle of frustrated total reflection.
It is further advantageous when a directional component of the radiated illumination beam path and of the imaging beam path is oriented parallel to the direction of height HA of the scanning surface. This satisfies an important criterion for the user-friendly arrangement of a scanning device.
It is also possible to couple the recording prism with a heating device 46 by means of which the support surface for the hand or fingers is preheated to a temperature that prevents condensation of the transpiration of the skin in a cool environment.